1. Field of the Invention
The present invention relates to a fusing processing method making use of Joule heat and a pressurization force in order to crimp a workpiece.
2. Description of the Related Art
An exemplary fusing work is illustrated in FIGS. 3A to 3C. This work provides electrical and physical connections between a covered wire 10 and a strip-like terminal 12 made of e.g., copper or copper alloy.
Referring first to FIG. 3A, a workpiece is inserted between a pair of (e.g., upper and lower) electrodes 14 and 16, the workpiece consisting of the terminal 12 and the covered wire 10 embraced in a hooked portion or a bent portion 12a of the terminal 12. The undersurface of the terminal hooked portion 12a is carried by the lower electrode 16 at a fixed position, with the upper electrode 14 abutting against the top surface of the terminal hooked portion 12a so that the latter is pressed downward with a predetermined pressurizing force F by a pressure device not shown. At the same time, a predetermined voltage is applied to the two electrodes 14 and 16 by a power supply apparatus not shown.
Then, current I first flows, through the terminal hooked portion 12a providing a current path, between the pair of electrodes 14 and 16, to generate Joule heat at the terminal hooked portion 12a. As a result of this, an insulator 10aof the covered wire 10 melts by Joule heat and peels off a conductor 10b as illustrated in FIG. 3B.
Once the insulator 10a is removed, current I is allowed to flow, through the conductor 10b (typically, made of copper) of the covered wire 10, between the two opposing electrodes 14 and 16 as illustrated in FIG. 3C. During the current-supplying period as well, the pressurization force F is still continuously applied to the two electrodes 14 and 16, and hence Joule heat and pressurization force F act, in cooperation, to cause the terminal hooked portion 12a and the covered wire conductor 10b to be integrally pressure welded or pressure squashed for crimping. This enables the covered wire 10 and the terminal 12 to be electrically and physically joined together in a rigid fashion. Due to an extremely small resistance of the conductor 10b of the covered wire 10 and the terminal 12, no nugget will be generated therebetween.
FIG. 6 illustrates a circuit configuration of a single-phase AC power supply apparatus that has hitherto been used for the fusing work as described above. FIG. 7 illustrates waveforms of the voltage and current delivered from the power supply apparatus.
In this power supply apparatus, a single-phase AC voltage V of a commercial frequency fed to input terminals 100 and 102 is applied to a primary coil of a step-down transformer 108 by way of a contactor that is comprised of a pair of thyristors 104 and 106. An AC induced electromotive force (secondary voltage) generated at the secondary coil of the transformer 108 is applied through the secondary conductor and the electrodes 14 and 16 to the workpiece W (10, 12) so as to allow a secondary current i2 having a larger current value than that of a primary current i1 to flow as the fusing current I through the secondary circuit.
The magnitude (effective value) of the fusing current I (i2) is determined depending on a conduction angle. Due to the presence of a substantially fixed relation between a firing angle and the conduction angle, it may be said that the magnitude depends on the firing angle. This power supply apparatus provides a control of firing angles (firing timings) xcex8 of the thyristors 104 and 106 by way of a firing circuit 112, to thereby control the effective value of the fusing current I (i2).
FIG. 8 illustrates a configuration of a DC inverter power supply apparatus that has hitherto been used in the fusing work. FIGS. 9A and 9B depict waveforms of the voltage and current output from the power supply apparatus.
This power supply apparatus comprises an inverter circuit 120 to which a DC voltage E is applied at a predetermined voltage level by a rectifying circuit not shown. The inverter circuit 120 includes switching elements and serves to issue high-frequency AC pulses in such a manner as to chop up the DC input voltage E at a high-frequency switching in response to a control pulse CP from a inverter control unit 128. The AC pulses output from the inverter circuit 120 are fed to a primary coil of a step-down transformer 122 so that AC pulses similar to those at primary side are acquired in the secondary coil. The secondary pulsed alternating current is converted into a direct current by a rectifying circuit 126 consisting of a pair of diodes 124a and 124b, with the secondary direct current i2 being fed as a fusing current I to the workpiece W (10, 12) by way of the electrodes 14 and 16.
In such a conventional fusing processing method using the single-phase AC power supply apparatus, the ratio is small of the effective current-supplying time (the time during which current actually flows) to the gross current-supplying time, so that a current peak value needs to be increased in each current-supplying cycle if it is desired to supply a sufficient thermal energy for the fusing work. However, the increased current peak value tends to result in an increased instantaneous peak value of Joule heat generated in the workpiece, which may possibly cause undesirable deformations or damages as a result of heat shock to which the workpiece W may be subjected. In the example of FIGS. 3A to 3C, immediately after the commencement of current supply (i.e., at the stage of FIG. 3A), the bend of the hooked portion 12a of the terminal 12 may crack in the vicinity of its top due to the heat shock.
On the contrary, in the conventional fusing processing method using the DC inverter power supply apparatus, the ratio of the effective current-supplying time is large and its heat generating efficiency is high, so that a sufficient thermal energy can be supplied to the workpiece even at a relatively low current peak value, and thus any heat shock can be suppressed. However, this method is problematic in that since the fusing current I can flow between the two electrodes only in the same direction (polarity), the amount of heat generation may differ from place to place due to Peltier effect appearing between the electrodes 14, 16 and the workpiece W, whereupon the deformations and wears at the extremities of the electrodes are apt to concentrate in one electrode (typically, in the electrode 14 at positive side), which may result in a cumbersome maintenance and a rise in cost.
The present invention was conceived in view of the above problems. It is therefore the object of the present invention to provide a fusing processing method capable of preventing any heat shock on a workpiece to improve the work quality and evening out the wears and degradations of the electrodes to improve the maintenance (workability, costs).
According to an aspect of the present invention, in order to attain the above object, there is provided a method of fusing a workpiece in which a pair of electrodes are pressed against the workpiece while simultaneously a current flows through the pair of electrodes to the workpiece to generate Joule heat, the method comprising the steps of converting an AC voltage of a commercial frequency into a DC voltage by means of a rectifying circuit; converting the DC voltage output from the rectifying circuit, into a pulsed voltage of a high frequency by means of an inverter; passing the high-frequency pulsed voltage output from the inverter through a transformer, to apply it via the pair of electrodes to the workpiece without any rectification at secondary side of the transformer; and segmenting a current-supplying time for a single fusing processing into a plurality of current-supplying periods, to output the high-frequency pulses with one polarity from the inverter in odd-numbered current-supplying periods, but to output the high-frequency pulses with the other polarity from the inverter in even-numbered current supplying periods.
In a fusing processing method of the present invention, the inverter allows a high-frequency waveform-controlled current to flow between two electrodes in each current-supplying period, whereby it is possible to achieve a high heat generating efficiency and hence to supply a sufficient heat energy to the workpiece even with a relatively low current peak value. This prevents the workpiece from being subjected to any heat shocks, enabling a stabilized fusing quality to be acquired. Furthermore, the polarity (direction) of current flowing between the two electrodes during the current-supplying time is inverted for each current-supplying period so that Peltier effect appearing between the electrodes and the workpiece is cancelled out or evened out and the amount of heat generation is also uniformed, thereby preventing deformations and wears at the electrode extremities from concentrating in one electrode, to ensure a fine finish in the fusing processing.